This document discusses different theories of acids and bases: 1) Arrhenius theory defines acids as substances that produce H+ ions in aqueous solution and bases as substances that produce OH- ions. Neutralization occurs via reaction of H+ and OH- to form H2O. 2) Bronsted-Lowry theory defines acids as proton donors and bases as proton acceptors. Acid-base reactions involve proton transfer. 3) Lewis theory defines acids as electron pair acceptors and bases as electron pair donors. Lewis acid-base reactions involve sharing of electron pairs to form adducts. Common examples include formation of hydronium ion and metal-ligand complexes.

1. Mr. H.N.SINGH
School of Pharmacy
Sharda University
Gr.Noida
ACID –BASE CONCEPTS

2. ARRHENIUS THEORY for ACIDS and
BASES
 Proposed in 1884 by Svante Arrhenius
 ACIDS: An acid is a substance that contains hydrogen and
produces H+ in aqueous Solution.
HCl (aq) H+ + Cl-
 BASES: A base is a solution that contains ,the OH group
and produces Hydroxide ions OH-, in aqueous solution.
 Ca(OH)2 Ca+2 + 2 OH-
 Neutralization is defined as the combination of H+ ions with
OH- ions to form H2O molecules.
 H+ (aq) + OH-(aq) H2O(l)

3. Proposed in 1923 by N. Bronsted and T.M.Lowry
Acid – any substance donating a proton, H+
Base – any substance accepting a proton.
 An acid-base reaction is the transfer of a proton
from an acid to a base.
 The complete ionization of hydrogen chloride, HCl
in water is an acid-base reaction.
 Step 1: HCl(aq) H+(aq) + Cl-(aq) (Arrehnius)
 Step 2: H+ (aq) + H2O(l) H3O+
 Overall: HCl(aq) + H2O(l) H3O+(aq) + Cl-(aq) (LB)
Bronsted-Lowery Theory of Acids and
Bases

4. Overall: HCl(aq) + H2O(l) H3O+(aq) + Cl-(aq) (LB)
acid conjugate
acidbase
conjugate
base
Overall: NH3(aq) + H2O(l) NH4
+(aq) + OH-(aq) (LB)
base conjugate
acidacid
conjugate
base
Whether water can act as an acid or base depends
on the other species present

5. Lewis theory
 Proposed by G. N. Lewis
An acid is any species that can accept a share in an
electron pair. A base is any species that can make
available or donate a share in an electron pair.
Lewis base - an electron pair donor
Lewis acid - an electron pair acceptor
Lewis acid + Lewis base  Adduct

6. Lewis Acids & Bases
A Lewis acid and base can interact by
sharing an electron pair.

7. Lewis Acids & Bases
A Lewis acid and base can interact by
sharing an electron pair.
Formation of hydronium ion is an excellent
example.
H
H
H
BASEACID
O—H
••••••
O—H
H
+
+

8. Other good examples involve metal ions.
Such bonds as the H2O ---> Co bond are often
called COORDINATE COVALENT BONDS
because both electrons are supplied by one of
the atoms of the bond.
H
H
Co
2+ ••
BASEACID
O—H
••••••
O—H
Co
2+
Lewis Acids & Bases

9.  SOME EXAMPLES OF LEWIS ACID AND BASE:
 1. LEWIS ACID: H+, NH4
+, Na-, K+, CN+2, Al+3 etc.
 2. LEWIS BASE: NH3, H2O, OH-, Cl-, CN- etc.